A speaker driver is a transducer that converts electrical signals into sound. That conversion occurs when a flexible membrane or diaphragm is induced to vibrate at audible frequencies. The vibrations thus produced propagate through the air as sound waves.
Tweeter loudspeakers vibrate most efficiently at high audio frequencies, typically between 2,000 Hz and 20,000 Hz (the upper limit of human hearing). The sound emanating from tweeters is often called “treble,” and this type of speaker derives its name from the sounds typically made by birds. Woofer loudspeakers vibrate most efficiently at low audio frequencies, typically between 40 Hz and 500 Hz. The sound emanating from woofers is often called “bass,” and this type of speaker derives its name from the sounds typically made by large dogs (“woof”).
In a normal speaker driver, the sound waves emanating from the front of the driver are 180 degrees out of phase with the sound waves emanating from the rear. The resulting unwanted sound cancellation is the reason that most speakers are fitted in enclosures. Enclosing a speaker practically isolates the sound emanating from the front of the speaker from that emanating from the rear.
However, even an enclosed speaker system can produce unwanted noise. The problem is far greater with woofers than with tweeters. When the volume of the sound is minimal, the noise may not be noticeable. But, as the volume increases, a buzzing sound appears. The buzzing sound is produced by sympathetic vibration of the speaker enclosure. It can even be worsened when the surface upon which the speaker system rests vibrates sympathetically.
The problem is even more pronounced in vinyl record players (phonographs) that have built-in speakers. A record player has a rotating turntable (upon which the record sits) and a cantilevered tone arm, which further comprises amplification electronics, a needle, and a magnetic or piezoelectric transducer, commonly referred to as a cartridge. Record players built during the last half of the Twentieth Century used diamond-tipped needles. In a record player, the small needle fits inside the groove of the rotating record. Variations in the topography of the groove cause the needle to vibrate. The cartridge translates the vibrations into electrical signals that propagate through the amplification system to the speaker. As the sound is produced, the phonograph chassis vibrates sympathetically. These vibrations are picked up by the cartridge and ultimately transmitted to the speakers. This is the phenomenon of feedback. One solution to the problem is remotely mounting speakers a distance away from the turntable. The problem is less severe in modern CD players, which read a disk with a laser, or in more modern sound systems that do not have moving parts. Indeed, vinyl records have been largely replaced by digital media, and phonographs by CD and MP3 players. However, many music lovers remain loyal listeners to vinyl records, and there is a demand for phonographs with built-in speakers.
The phonograph was invented by Thomas Edison during the last part of the Nineteenth Century. During the early Twentieth Century, wax cylinder recordings were replaced by flat record disks. As sound quality became more important to music listeners, phonograph technology began to evolve. In 1946, U.S. Pat. No. 2,325,807 was issued to I. L. Stephan for a phonograph mounting that included a floating base supported by a plurality of conical compression springs. Stephan's purpose was to isolate his turntable from vibrations of the phonograph chassis. In 1956, U.S. Pat. No. 2,775,309 was issued to Villchur for a high quality speaker system. Loudspeakers produced according to Villchur's patent were widely sold by Acoustic Research, a Cambridge, Mass. company. These AR speaker systems used a different support for their diaphragms and used the elasticity of air within an enclosure of 1.7 cubic feet to provide the restoring force to the woofer cone. Acoustic Research referred to this technology as “air suspension.” While AR speaker systems yielded higher quality bass reproduction, these speakers needed to be remotely located from the record turntable.
In 1967, U.S. Pat. No. 3,342,498 was issued to R. G. Eberhardt for a portable phonograph having integral speakers. Eberhardt placed his speakers in a five-sided speaker assembly that was mounted in the phonograph by a plurality of coiled compression springs. His objective was to avoid acoustic feedback by damping the vibration of the speakers from the turntable. While this may have provided some improvement in sound quality, it was largely unsuccessful for three reasons. First, the speaker assembly was open-ended, thus failing to isolate the low frequency waves emanating from the front of the speaker from those emanating from the rear. Second, although Eberhardt claimed otherwise, acoustic vibrations were transmitted to the phonograph chassis because the use of bulky components necessitated supportively heavy coiled compression springs. Conduction of sound waves through the chassis has a greater tendency to produce unwanted feedback than sound convection through the air. Third, although the speaker assembly itself became a bass-reinforcing resonator, it did so at the expense of distortion performance.
In 2016, U.S. Pat. No. 9,473,853 was issued to H. M. Huang for a compact, lightweight, and airtight speaker module. Placing Huang's speakers in a small volume sealed container forces air pressure onto the speaker cone, thus inhibiting vibration. The air in the container acts like a spring. When the sealed volume is small, the air has less elasticity. However, the container includes a passive radiator, which serves to increase compliance of the container interior, thereby reducing the size requirement. Thus, Huang's speaker module produces high quality bass and treble sound. However, it does not solve the vibration problem in a record player. If this speaker module is directly mounted to the phonograph chassis, sound waves conduct to the tone arm, and this conduction creates acoustic vibration and feedback.
There has been a long-felt need among vinyl record aficionados for a portable record player or phonograph with built-in speakers, which produces high quality sound without distortion.